1. Field of the Invention
This invention relates to an authentication system and, more particularly, to a system for authenticating a user located at a requesting node to an authenticating node coupled to the requesting node by a communications channel, using one-time passwords.
2. Description of the Related Art
Systems for authenticating users to system resources such as user applications are well known in the art. Many such systems operate by using passwords that are generally known only to the user and issuer or authenticator. Systems that transmit such passwords over a communications channel from a resource-requesting node (e.g., a workstation or ATM terminal) are vulnerable to the interception of transmitted passwords by unauthorized persons who may tap into the communications channel. Such persons may thereafter attempt to gain unauthorized access to system resources by using the intercepted passwords together with such nonsecret information as a user ID which may also have been intercepted.
To thwart such attempts to gain unauthorized access, passwords are usually transmitted over communications channels in encrypted form, so that access to the communications channel does not yield the password itself in plaintext form. However, even systems that encrypt passwords before transmission are vulnerable to the possibility that a person may record the encrypted password and thereafter inject the encrypted password directly into the communications channel (rather than through a terminal and encryption device) to secure the desired access.
To counter this more sophisticated challenge, recent systems have used so-called "one-time" or "dynamic" passwords that are valid for only a brief time interval (e.g., a minute or less), so that interception of such a password during one interval provides no useful information for gaining access to a system during a later interval. Authentication systems of this type are described in Vaughan U.S. Pat. No. 4,800,590 and Weiss U.S. Pat. No. 4,885,778, as well as in the commonly assigned, copending application of applicant Richard H. Guski, Ser. No. 08/012,087, filed Feb. 1, 1993, and entitled "Method and Apparatus for Securing Communications in a Distributed Data Processing System", the specifications of which are incorporated herein by reference. In the system described in the copending application, for example, a one-time password is generated by a requesting product or function (or by the host authentication system at the request of another program) and used instead of the user's regular host authentication password when an end user logs onto a host application or is otherwise authenticated by the host authentication system. It is a secure way to access host applications because the user's real host authentication password does not have to flow across the network in clear text, nor can the one-time password be reused if intercepted.
Systems of the type described in these references generate their one-time passwords as a function of secret information (such as a user password or an encryption key), time-dependent information such as a time-of-day (TOD) value or a time/date value, and, optionally, nonsecret information such as a user ID and application ID. The one-time password is transmitted to the authenticating node, where it is compared with a comparison password generated in a similar manner using the same secret information and nonsecret information, if any, together with a time value available at the authenticating node. If the transmitted password agrees with the comparison password, the user is authenticated and granted access to the system resource.
The system described in the above-identified copending application has a performance problem at the authenticating end, since the only way in that system to ensure that the incoming one-time passwords are valid and not intrusion attempts is to generate a corresponding one-time password and compare the incoming one-time password with the generated one. The generation of one-time passwords requires repeated uses of the DES encryption procedure, which is computationally intensive. This problem is further compounded because the input to the one-time password generation process involves time information as one of the input variables. Because no two computer clocks are ever set exactly the same and a delay can occur while the one-time password is in transit, multiple passes through the procedure for various time values centered about the current clock value of the validating computer are necessary.
The necessity for generating multiple comparison passwords would be avoided if the original time/date value could be recovered from the received one-time password for direct comparison with the time/date value available at the authenticating node. However, in the system described in the copending application, this is not possible. In that system, each byte of a 64-bit time-dependent encryption product (from which the original time/date information could be recovered) is converted to a corresponding alphanumeric character (A-Z, 0-9) of the password using a byte-to-character translation table. Since each 8-bit byte of the time-dependent encryption product may have one of 256 possible values, whereas each alphanumeric character may have one of only 36 possible values, the byte-to-character translation table necessarily uses a many-to-one mapping, so that neither the 64-bit encryption product nor the original time/date value may be recovered from the received password at the authenticating node. Therefore, it is necessary to generate comparison passwords from trial time values, as indicated above.
The second problem, also related to performance, is that the system described in the copending application generates one-time passwords that are indistinguishable from other possible password character combinations. That is to say, since each 8-character password could have been generated from any one of a large number of 64-bit time-dependent encryption products, no password can be trivially rejected. This means that when the authenticating node receives an invalid password, that password must be passed through the entire password procedure for each of the possible time values, which may number in the hundreds. The invalid password will fail validation, but much computational power is wasted in this process.